When designing microwave circuits, transmission lines and waveguides are commonly used. A transmission line is normally formed on a dielectric carrier material. Due to losses in the dielectric carrier material, it is sometimes not possible to use any transmission lines. When there, for example, is a diplexer in the layout, the diplexer may have to be realized in waveguide technology. Waveguides are normally filled with air or other low-loss materials.
Waveguide diplexers used today are large mechanical components screwed into a mechanical cabinet and connected to different parts such as for example, an antenna via some type of waveguide flange. It is desirable to mount such a diplexer structure on a dielectric carrier material to form a surface-mounted waveguide structure.
Such a surface-mounted waveguide is normally made having three walls and one open side. Metallization is then provided on the side of the dielectric carrier material facing the waveguide, where the metallization serves as the remaining wall of the waveguide, thus closing the waveguide structure when the waveguide is fitted to the dielectric carrier material.
An example of surface-mountable waveguides is disclosed in the paper “Surface-mountable metalized plastic waveguide filter suitable for high volume production” by Thomas J Müller, Wilfried Grabherr, and Bernd Adelseck, 33rd European Microwave Conference, Munich 2003. Here, a surface-mountable waveguide is arranged to be mounted on a so-called footprint on a circuit board. A microstrip conductor to waveguide transition is disclosed, where the end of the microstrip conductor acts as a probe for feeding the waveguide's opening.
Surface mounting of large mechanical components, such as diplexers, may result in mechanical stress problems due to different coefficients of thermal expansion, CTE, of the materials involved, such as, for example, so-called twist and bow. Furthermore, such a large surface-mounted structure as a diplexer is too large to handle in an automated production line.
One way to solve this problem is to split the diplexer into a number of smaller parts. These parts have to be sufficiently connected to each other in order to present a proper electrical function. This problem is apparent for all large surface-mounted waveguide structures.
An example of a solution according to prior art is disclosed in prior art FIG. 1, showing a simplified cross-sectional side-view. A first surface-mounted waveguide part P1 and a second surface-mounted waveguide part P2 are mounted on a dielectric carrier material P3. The ends of these surface-mounted waveguide parts that face each other comprise respective 90° bend parts P4, P5, changing the direction of the transmitted signals 90° such that they are directed through corresponding openings P6, P7 in the dielectric carrier material P3. On the other side of the dielectric carrier material, a third surface-mounted waveguide part P8 is mounted, the third surface-mounted waveguide part comprising two 90° bend parts P9, P10 positioned such that the signal directed through the openings P6, P7 is guided through the third surface-mounted waveguide P8 part in such a way that it functions as a link between the first surface-mounted waveguide part P1 and the second surface-mounted waveguide part P2. The details of the bend parts P4, P5; P9, P10 are not shown in FIG. 1, only the function is schematically indicated.
This solution is, however, rather complicated and requires that a special waveguide part, having two 90° bend parts, is mounted on the other side of the dielectric carrier material, and that all waveguide parts are aligned with the openings such that there is no interruption in the transmission of the signals.
Placing the waveguide parts as close to each other as possible always results in a small gap anyway, which limits the performance regarding losses and spurious signal components due to leakage of microwave radiation and mismatch.